A mission NASA is designing to probe the nature of mysterious dark energy could discover thousands of alien planets as well.

NASA's proposed Wide-field Infrared Survey Telescope (WFIRST) mission aims to help researchers better understand dark energy, the puzzling stuff that makes up about three-quarters of the universe and drives its accelerating expansion.

But WFIRST — which is tentatively scheduled to launch in the early to mid-2020s — should also prove to be an adept planet hunter, complementing the activities of the space agency's prolific Kepler space telescope, researchers say. [Gallery: A World of Kepler Planets]

"We predict WFIRST will have 3,000 individual planet detections, the same order of magnitude as Kepler," Scott Gaudi, of Ohio State University, said in April during the Space Telescope Science Institute's Habitable Worlds Across Time and Space Symposium in Baltimore.

Gravitational microlensing

Scientists detect planets around other stars using several different methods. Kepler notes the tiny, telltale dimming of light that occurs when a planet crosses, or transits, the face of its host star from the spacecraft's perspective. But WFIRST would rely on gravitational microlensing.

In this technique, astronomers watch what happens when a big object passes between Earth and a background star. The foreground object's gravity bends and amplifies the light from the background star, acting like a magnifying glass.

If the foreground object is a star, and it has planets, the planets can affect the magnified light, creating a signal that astronomers can detect. The process behind this strategy was laid out in 1936 by Albert Einstein, based on his general theory of relativity.

Earth-based telescopes have already detected more than 20 exoplanets using microlensing. WFIRST will be a space-based telescope, which opens up greater detection abilities, researchers said.

"If you go to space, you can do a lot of great things," Gaudi said.

Because microlensing requires the correct lineup of foreground and background stars, the ability to follow up on WFIRST's finds will be limited. However, the process will expand the population of known alien planets, aiding scientists aiming to determine how rare Earth-size planets might be.

"This will dramatically improve our yield of planets," Gaudi said.

A census of worlds

WFIRST should provide a wealth of information about what types of planets exist, allowing stronger statistical conclusions to be drawn, researchers said. Such work would be a nice follow on from Kepler, which has discovered thousands of candidate exoplanets, many of them in solar systems very different than our own.

"If every solar system looked like ours, Kepler would have found very few or no planets," Gaudi said. "The solar systems we're learning about with Kepler are very different from our own."

Kepler has had a great deal of success spotting planets that orbit relatively close to their stars (because they transit frequently). WFIRST, on the other hand, will be more sensitive to larger bodies farther from their suns, researchers said.

In addition, WFIRST should be able to detect smaller distant planets, as well as free-floating "rogue planets" that have been ejected from their systems. Together, Kepler and WFIRST will cover virtually the entire plausible spectrum of planets in mass and orbits.

WFIRST will be able to capture information about Earth-size planets that lie farther from their suns than Earth does, as well as unbound planets the size of Mars. According to Gaudi, in favorable cases, the instrument should be able to detect a terrestrial moon orbiting a distant Earth, or a gas-giant satellite as large as Ganymede (Jupiter's largest moon), though both observations would be challenging. Unbound moons, like unbound planets, would also be detectable.

Of the 3,000 new planets expected to be found by WFIRST, scientists think about 300 will be Earth-size worlds and 1,000 will be "super-Earths," possibly rocky planets up to 10 times the mass of our own. Such predictions are based on present-day understanding of the distribution of types of planets, knowledge that may be either strengthened or challenged by the wealth of data that WFIRST will bring.

With WFIRST, Gaudi said, "we'll measure the galactic distribution of the planets."

At present, the observatory is in the pre-formulation stage, where it will remain until 2016. In addition to creating a statistical catalog of exoplanets, WFIRST will also directly image previously confirmed planets, study black holes, and hunt for clues about dark energy.